[GlenTheMachine]

So one of the oldest known prion diseases is a disease of sheep called scrapie. It is not transmissible to humans, thankfully.

It turns out that there is genetic resistance to it. There's a specific gene, of which there are two forms, called Q and R. If a sheep is Q-Q, it has no resistance. If it is Q-R is has some resistance. If it is R-R it is completely immune.

All of which is to say: we should not just assume that exposure to prions results in a 100% disease rate, nor should we assume that prion disease itself has a 100% mortality rate. Because the diagnosis for prion disease is clinical. In other words, we only know someone has the prion if they show signs of the disease. And we only look for the disease once someone starts showing significant signs of disease. But there is no test that shows that you ave been exposed, nor is there a test that shows you have been infected but that your infection is spreading so slowly that (say) prostate cancer or heart disease will kill you long before the prion does.

https://en.wikipedia.org/wiki/Scrapie